Further speed improvements

- Store Emscripten cache inside node_modules/.em_cache. Docker image ships without LTO libs, so Emscripten has to rebuild stdlibs on every build otherwise.
 - Merge webp_enc + webp_dec build scripts. Core libwebp library is same in both cases, so there's no point in storing and building two copies of it.

codecs/webp/dec/webp_dec.cpp

@@ -0,0 +1,47 @@
+#include "emscripten/bind.h"
+#include "emscripten/val.h"
+#include "src/webp/decode.h"
+#include "src/webp/demux.h"
+#include <string>
+
+using namespace emscripten;
+
+int version() {
+  return WebPGetDecoderVersion();
+}
+
+class RawImage {
+public:
+  val buffer;
+  int width;
+  int height;
+
+  RawImage(val b, int w, int h)
+    : buffer(b), width(w), height(h) {}
+};
+
+uint8_t* last_result;
+RawImage decode(std::string buffer) {
+  int width, height;
+  last_result = WebPDecodeRGBA((const uint8_t*)buffer.c_str(), buffer.size(), &width, &height);
+  return RawImage(
+    val(typed_memory_view(width*height*4, last_result)),
+    width,
+    height
+  );
+}
+
+void free_result() {
+  free(last_result);
+}
+
+EMSCRIPTEN_BINDINGS(my_module) {
+  class_<RawImage>("RawImage")
+    .property("buffer", &RawImage::buffer)
+    .property("width", &RawImage::width)
+    .property("height", &RawImage::height);
+
+  function("decode", &decode);
+  function("version", &version);
+  function("free_result", &free_result);
+}